1. Technical Field
This invention relates to a process of plating tin or tin-alloy on a metal substrate. In particular, the present invention relates to a negative electrode comprising tin or tin-alloy active material and a rechargeable lithium battery using the same.
2. Description of Related Prior Art
Rechargeable lithium-ion batteries are the main power sources for many portable electronic devices with high demand for high energy density and weight saving. Currently, these batteries are based on a carbonaceous anode material, which has a limited theoretical capacity of 372 mAh/g. Lightweight lithium metal is known to provide much higher energy density, however, use of it as the anode is unsafe due to the unavoidable formation of lithium dendrites during charging, which results in electric short-circuit of the battery. Furthermore, high reactivity of metal lithium to the electrolytic solvents results in fast degradation of cell chemistry, which hence limits cycle life of the batteries. To alleviate these problems of metal lithium, Li alloys have been proposed as an alternative anode material of rechargeable lithium batteries. Among numerous Li-alloys for the anode material of rechargeable lithium batteries, Li—Sn alloy has shown most promising as described in J. Electrochem. Soc. 128 (1981) 1181 and reviewed in Electrochim. Acta 45 (1999) 31. The Li—Sn alloy is generally fabricated in a form of sheet and it is directly used as both the anode active material and current collector. Owing to huge volume changes accompanied during alloying and dealloying, the alloy quickly pulverizes and loses electric contact between the alloy particles. As a result, the performance of the Li alloy fades very fast with cycling. An easy solution to retaining good electrical contact of the alloy particles is to coat tin or tin-alloy on a metal substrate as the current collector at a small expense of energy density due to the incorporation of electrochemically inert agents such as binder and conducting additive. Therefore, a technique without need of binder and conducting additive based on plating of alloy active material on the current collector is highly recommended for maximized energy density of an alloy anode.
Electroplating and electroless plating are two widely used methods for plating tin or tin-alloy on a metal substrate. The electroplating typically uses an aqueous acidic bath containing metal ions to be plated with an external current applied on two ends of the metal substrate to be plated and a counter electrode. The desirable acidity of the solution is achieved by adding appropriate amount of either inorganic acid as disclosed in U.S. Pat. Nos. 3,769,182 and 4,118,289 or organic acid as disclosed in U.S. Pat. Nos. 4,565,610, and 4,617,097. To facilitate the plating process, various complexing agents and surfactants ranging from small organic molecules to large polymeric molecules are used as the auxiliary additives as disclosed in U.S. Pat. Nos. 4,565,609, 4,599,149, 4,701,244, 4,849,059, 5,174,887, 5,282,954 and 7,160,629. Disadvantages of the electroplating process are: it is not suitable for plating of large size substrates and it is difficult to make a uniform plating layer due to uneven distribution of current density, especially when the current and thickness are high. On the other hand, metal tin or tin-alloy as an anode material of rechargeable battery is required to be highly porous so as to buffer the huge volume change accompanied with cycling for the durable cycle life. It has been reported (Adv. Mater. 12 (2003) 1610-1614) that such structures can be obtained by adjusting electroplating conditions such as current density and acidity.
Electroless plating, or immersion plating, involves the use of a plating bath without the imposition of external electric current where the substrate is plated by reduction of the metal ions from a solution of the plating metal salt. Electroless plating is characterized by a catalytic nature of the substrate surface which enables the metal to be plated to any thickness. Typically, the plating solution comprises a solvent, a salt of the plating metal, a reducing agent capable of reducing the metal ions to be plated, a complexing agent for the metal ions to be plated, a surfactant as the wetting agent, and an appropriate amount of acid as a pH regulator of the solution. In the plating process, the reduction is catalyzed by the surface of the substrate to deposit the reduced metal onto the substrate. Since the reduction is autocatalytic without need of an external current, the electroless plating is suitable for making a plating layer with any thicknesses and excellent uniformity. A number of inorganic and organic compounds have been used as the reducing agent for the electroless plating of tin and tin-alloy. These compounds include alkali metal polysulfides as disclosed in U.S. Pat. No. 4,027,055, hypophosphorus acid and water soluble hypophosphites as disclosed in U.S. Pat. Nos. 4,093,466, 4,194,913, 5,248,527, 5,266,103, as well as guanamine compounds as disclosed in U.S. Pat. No. 4,555,314.
U.S. Pat. Nos. 4,550,037 and 4,618,513 taught a method for coating a tin layer on an electro-conductive substrate by an immersion process. In this method, metallic zinc was used as the reducing agent, and many strips of zinc were directly contacted with the substrate to be coated or indirectly connected the substrate to be coated using copper wires while both were immersed in the bath containing a stannous salt and a special surfactant. This process is capable of coating a pore-free, smooth and highly adherent tin layer on the electro-conductive substrate. However, this process is not suitable for plating of tin on a substrate with large area, and the thickness of tin coating is highly dependent on the positions of contacting points or connecting points between the zinc and substrate. In addition, pore-free structure is not favorable for long cycle life of rechargeable lithium battery when tin is used as the negative electrode.